Most conventional wafer polishing machines involve a table-type support having a rotatable polishing surface to which a polishing pad is mounted. The polishing pad is opposed by a rotatable polishing head to which a wafer carrier is mounted. The wafer is adhered to the carrier with the wafer face to be polished exposed. (In some prior patents or other publications, the carrier is referred to as a sub-carrier.) A wet polishing slurry, usually comprising a polishing abrasive suspended in a liquid, is applied to the polishing pad. The polishing head, including the carrier with adhered wafer, is moved to bring the exposed face of the wafer into contact with the wet polishing pad, for polishing. Downward polishing pressure is often applied between the rotating wafer and the rotating polishing pad during the polishing operation.
After the face of the wafer has been polished the wafer is picked up and removed from the wet polishing pad. It is desirable to have the wafer release from the polishing pad and remain attached to the carrier when the polishing head is lifted away from the polishing pad, without requiring the polished wafer face or the wafer edges to be contacted. Available materials and methods for adhering a wafer to a carrier do not always provide sufficient adhesion to reliably retain the wafer on the carrier against the strong adhesion between the wafer and the polishing pad.
The adhesive force between the wet pad and the wafer after polishing can be quite large even though the wafer is relatively lightweight. The smooth surface of the polished wafer, the presence of pores on the surface of many types of polishing pads which act as miniature suction devices, the presence of the fluid slurry which enhances the suction holding action of the pores, and the downward pressure often applied during polishing all tend to create a strong adhesion between the wafer and the polishing pad.
The conventional apparatus and methods do not provide sufficient adhesion between the carrier and the wafer to overcome the strong suction force holding of the polished wafer to the wet polishing pad, and the wafer undesirably remains adhered to the pad. When the wafer is not retained on the carrier, the wafer is usually manually removed from the polishing pad.
One conventional method of holding the wafer to the carrier uses an adhesive insert, such as a poromeric insert, between the carrier and the wafer. However, the adhesive force provided by such inserts may be insufficient to retain the wafer on the carrier. For example, some recently developed polishing pads, such as the IC1000 polishing pad made by Rodel (9495 East Salvador Drive, Scottsdale, Ariz., 85258), adhere the polished wafer to the pad particularly strongly after polishing and when one is used it is not unusual for a polished wafer to remain adhered to the pad when the polishing head is lifted away.
Heretofore, there have been some attempts to utilize a vacuum force, rather than an adhesive insert, to hold a wafer to a carrier. Japanese Patent JP 62-124844, for example, suggests the use of a vacuum holding force which is applied to a wafer through a porous ceramic carrier. The porous structure of the ceramic material communicates the vacuum pressure uniformly to a surface of the carrier that mates to a back face of the wafer. Patent JP 62-124844 also suggests the use of a pressurized fluid to release the wafer from the carrier. Other attempts to use vacuum force to adhere a wafer to a carrier have also been made. U.S. Pat. No. 4,193,226, for example, suggests the use of vacuum force applied to a wet absorbent material insert in contact with the entire wafer surface to adhere the wafer to the polishing head. It also suggests the use of positive fluid pressure including air and water to assist in releasing the wafer from the carrier.
The prior attempts to solve the problem of reliably releasing a wafer from a polishing surface and retaining it on the polishing head wafer carrier have not been entirely satisfactory because they do not provide for reliable release of the wafer from the pad and retention on the carrier. In particular, the prior solutions do not meet the needs of automated processing where robotics technology necessitates more reliable apparatus and methods for releasing the wafer from a wet polishing pad. Automation also requires the removal of polishing residues from the wafer carrier after each polishing operation so that the next wafer may be mounted without interference or distortion.